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Thread: Prism Problems. Suspect autoblockers

  1. #1
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    Confused Prism Problems. Suspect autoblockers

    We're having a rash of prism. I suspect our autoblockers. Current SOP is to run hourly test trays, inspect at QC. If they fail we calibrate. Turn around time is an issue, as well as accuracy at QC.

    I acquired DACs Prism Detector Tools Item DV-PR-DT) from a dusty drawer. I like the idea of changing our SOP to take direct measurements of the blocking ring, not relying on a 65-75 minute turn around and a subjective lensometer reading.

    I'm having difficulty translating our tolerances to the dial indicator reading of the measurement surface of the prism detector ring. I have the dial indicator set to sweep a 60mm diameter of the prism detector tool.

    I meticulously calibrated a blocker. Blocked up the test lens and swept across it with the indicator. Setting the low point to 0.00 it went up to just a hair over 0.01. Recalculated the job to block 0.12dpt in OU. Swept it with the indicator and it swung 0.00-0.24.

    Can someone help me derive whether this is accurate? Or can share their SOPs with this tool?

    Thank you.

  2. #2
    OptiWizard
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    Need more information about the problem. how much prism are you talking about. Is it on all materials and Rx’s is it always in the same direction.

  3. #3
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    Quote Originally Posted by Lensman11 View Post
    Need more information about the problem. how much prism are you talking about. Is it on all materials and Rxs is it always in the same direction.
    All materials, lens styles. Random directions. Error seems to accumulate over time. Gets up to around 0.50dpt before it seems to get caught, hence trying to transition over to a more direct measurement of the blocking surface via the dac test lenses. Optotech OTB 80 A2

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    I would check chill ring temps and determine how quickly temps recover after blocking. You mentioned an increase over time which might be an indicator of chill ring becoming warmer over time. Where the lenses with increase unwanted prism block later then the lenses with no prism? Any changes in ambient lab temp?

    Increasing set time may help. Decrease eject pressure.

    My thoughts are that blocking material (assuming alloy) is not hardening quickly enough so when block/lens ejected the block is being "pushed" into blocking material.

  5. #5
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    Quote Originally Posted by PRECISIONLAB View Post
    I would check chill ring temps and determine how quickly temps recover after blocking.
    Thank you. This was my first attack. Each blocker has its own independent chiller set to 10c. Cool down times are set at 30 seconds. Our previous prism Problems were from a certain operator setting the cool down times to as low as 10 seconds in some asinine self-appointed directive to make them "go faster". This current problem is across all shifts and operators. Blocking ring and cores have been checked via laser thermostat to be consistent over time. Ambient temperatures are stable +/-3f around 73f

    I do not suspect a temperature problem as when prism is finally detected with the lensometer at QC I have personally gone to the blockers, put the cores in the home position and measured them with the blocking ring indicator and they are out of alignment and require re-zeroing.

    The issue is that by the time the prism is detected hundreds of jobs have already been blocked.

  6. #6
    OptiWizard
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    Just zero in on one blocker if you feel that is the problem. Get a piece of plastic or glass which is flat on both sides and a perfect Plano lens. You can check it in the lensometer or with your dial indicator. Block it in the normal manner and measure it again with your Dac device if the blocker is the problem it will be obvious at this point.

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